[lld][cmake] Fix LLVM_LINK_LLVM_DYLIB build

Summary:
Lld's build had a couple of issues which prevented a successfull
LLVM_LINK_LLVM_DYLIB compilation.

- add_llvm_library vs llvm_add_library: One adds a library to libLLVM.so, other
  one doesn't. Lld was using the wrong one, causing symbols to be mupltiply
  defined in things linking to libLLVM.
- confusion when to use LINK_LIBS vs LINK_COMPONENTS in llvm_add_library
- not using LLVM_LINK_COMPONENTS for add_lld_tool

With these fixes lld compiles and it's test suite passes both in
LLVM_LINK_LLVM_DYLIB mode and without it.

Reviewers: ruiu, beanz

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28397

llvm-svn: 291432